1. Field of the Invention
The present invention is directed to a method for extending the cutoff wavelength of Indium Galium Arsenide (InGaAs) primarily for spectroscopy and night vision applications by increasing the In content by the use of highly strained InGaAs. Highly strained InGaAs quantum wells are created by balancing compressive strain with tensile strain. A lattice matched intermediate layer can be used to transition from a compressive strained layer to a tensile strained layer. The present invention also relates to a semiconductor device having a strain compensated banier region.
2. Related Art
The paper by Dries, et al., STRAIN COMPENSATED In.sub.1-x Ga.sub.x As (x&lt;0.47) ARSENIDE QUANTUM WELL PHOTODIODES FOR EXTENDED WAVELENGTH OPERATION, published in Applied Physics Letters, Volume 73, p. 2263, the entire disclosure of which is incorporated herein by reference, discusses experiments forming p-i-n PHOTODIODES using a strain compensated multiple quantum well absorption region. These detectors exhibit high quantum efficiencies (QE), low dark currents, and are potentially useful in night vision and spectroscopic applications as well as any other suitable applications.
Present technologies for the extension of InGaAs into the .lambda.=2 .mu.m wavelength band utilize buffer layers of lattice-mismatched InAsP which produce laterally threading misfit dislocations. This threading mechanism reduces the effect of lattice-mismatch on subsequent epitaxial layers. The InGaAs detectors fabricated using this technique can have responses to a wavelength of 2.5 .mu.m, but suffer from large dark currents due to residual defects in the epitaxial layers.
Applications for night vision, remote sensing and spectroscopy have increased interest in the 1.65 .mu.m to 2 .mu.m wavelength band. New detectors and detector materials with access to this range of wavelengths are particularly desirable due to the limited utility of HgCdTe, InAs, InSb, and strain-relaxed InGaAs based devices. HgCdTe is plagued by material growth issues and the narrow bandgaps of InAs and InSb result in detectors with large dark currents at room temperature. Furthermore, GaInAsSb devices grown on GaSb substrates have dark currents in the microamp range for detectors as small as 100 .mu.m in diameter. Lattice-mismatched InGaAs, when grown on buffer layers of relaxed InAsP, results in detectors with acceptable dark currents and high bandwidth. However, residual defects in the epitaxial layers, as well as the lack of integration capability with InP electronics, highlight the need for novel materials and detectors for use at wavelengths .lambda.&gt;1.65 .mu.m.
In order to increase the cuttoff wavelength, it is necessary to increase the In content. The thickness of the absorption region is limited by critical thickness considerations, and thus quantum wells are formed. This is deleterious to the prospect of extending the absorption region to longer wavelengths, for the quantum confinement produces a blue shift from the bulk band edge. The cutoff wavelength is reduced to a lesser extent by strain in the InGaAs, for compressively strained InGaAs has a larger bandgap than relaxed InGaAs. Previously, the use of strained InGaAs for detection at .lambda..ltoreq.2 .mu.m has been demonstrated in separate absorption, multiplication layer avalanche photodiodes (D. Gershoni, H. Temkin, and M. B. Panish, Appl. Phys. Lett. 53, 1294-1296, (1988)), but critical layer thickness considerations limited these devices to 10 quantum wells (QW), resulting in low quantum efficiencies. The present invention offsets the compressive strain in the QW by an equal and opposite strain in the barrier region surrounding the QW. Such strain-compensation techniques, in principle, permit the growth of an unlimited number of strained quantum wells, thus dramatically improving device quantum efficiency.
The additional degree of freedom for bandgap engineering afforded by strained layer materials has been used to advantage in achieving high performance lasers, detectors, and modulators. The present invention uses strain-compensated InGaAs grown on InP substrates in very low dark current multiple quantum well (MQW) p-i-n diodes for efficient detection of light to wavelengths as long as .lambda.=2.1 .mu.m.